 //声明省
 var pres = ["北京", "上海", "山东"]; //直接声明Array
 //声明市
 var cities = [
   ["东城", "昌平", "海淀"],
   ["浦东", "高区"],
   ["济南", "青岛"]
 ];
 var areas = [
   [
     ["东城1", "东城2", "东城3"],
     ["昌平1", "昌平2", "昌平3"],
     ["海淀1", "海淀2", "海淀3"]
   ],
   [
     ["浦东1", "浦东2", "浦东3"],
     ["高区1", "高区2", "高区3"]
   ],
   [
     ["济南1", "济南2"],
     ["青岛1", "青岛2"]
   ]
 ]
 //设置一个省的公共下标
 var pIndex = -1;
 var preEle = document.getElementById("pre");
 var cityEle = document.getElementById("city");
 var areaEle = document.getElementById("area");
 //先设置省的值
 for (var i = 0; i < pres.length; i++) {
   //声明option.<option value="pres[i]">Pres[i]</option>
   var op = new Option(pres[i], i);
   //添加
   preEle.options.add(op);
 }

 function chg(obj) {
   if (obj.value == -1) {
     cityEle.options.length = 1;
     areaEle.options.length = 1;
   }
   //获取值
   var val = obj.value;
   pIndex = obj.value;
   //获取ctiry
   var cs = cities[val];
   //获取默认区
   var as = areas[val][0];
   //先清空市
   cityEle.options.length = 1;
   areaEle.options.length = 1;
   for (var i = 0; i < cs.length; i++) {
     var op = new Option(cs[i], i);
     cityEle.options.add(op);
   }
   for (var i = 0; i < as.length; i++) {
     var op = new Option(as[i], i);
     areaEle.options.add(op);
   }
 }

 function chg2(obj) {
   var val = obj.selectedIndex - 1;
   var as = areas[pIndex][val];
   areaEle.options.length = 1;
   for (var i = 0; i < as.length; i++) {
     var op = new Option(as[i], i);
     areaEle.options.add(op);
   }
 }